Hybrid aircraft
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Hybrid Aircraft
Lockheed Martin Advanced Development Projects is reportedly making perhaps the first realistic tests of a hybrid aircraft--a concept that dates back many decades but that is just now being tried at a significant manned scale. The Skunk Works had secretly built the craft and hoped for a quiet first flight at its Palmdale, Calif., facility, but a few passers-by noticed the strange object in the sky.
The Defense Dept. is showing interest in two categories of LTA related technologies--those that can carry large cargo at low altitude, exemplified by the Defense Advanced Research Projects Agency (DARPA) Walrus program, and those that can operate in high-altitude low-wind conditions and remain on station for long periods of time. The configuration of the Skunks Works ship indicates it is the former--a hybrid heavy-load carrier hybrid aircraft.
A hybrid aircraft derives most of its lift by being filled with a lighter-than-air gas such as helium. Overall, it is heavier than air and gains the final 20% or so of lift by flying like an aircraft, but with slow takeoff and landing speeds that allow operations from short unprepared strips. The Skunk Works made the first flight of its "P-791" testbed on Jan. 31 at its facility on the Palmdale Air Force Plant 42 airport. The manned flight was about a 5-min. circuit around the airport in the morning and appeared to be successful. The company did not announce or want to discuss the flight.
The P-791 is not part of a government contract, but rather an independent research and development project by the Skunk Works to better understand hybrid aircraft capabilities and technologies, such as materials, a company official says. However, it may also be a quarter-scale prototype of a heavy-lifter. TO GAIN MORE SPAN TO ACT LIKE a wing, the P-791 is three pressurized lobes joined together. An observer of the first flight says it was about the size of three Fuji blimps blended together. The Fuji blimp, a Skyship 600 model, is 206 ft. long. That suggests the P-791 would have a gross lift of roughly 3-5 tons.
The observer saw the craft performing very tight 360-deg. turns while taxiing. It made a brief takeoff roll, climbed to a low altitude, made a few banks--including a long sweeping turn--then came back and landed. The landing approach had a nose-down body attitude that levelled for the flare. The flight was very smooth, the observer says. The craft was flown by P-791 Chief Test Pilot Eric P. Hansen.
The speed of the testbed was estimated at about 20 kt. A full-scale version would be able to go much faster, over 100 kt. Lockheed Martin has long proposed a large transport hybrid aircraft, at one time called the Aerocraft, which was halted around 2000 (AW&ST Feb. 22, 1999, p. 26). That design was about 800 ft. long and was to carry 1-1.2 million lb. at 125 kt. The Skunk Works was one of two contractors to receive one-year, $3-million Darpa contracts in August 2005 to study Walrus.
Hybrid aircraft have a long history. The Aereon Corp. in New Jersey started experiments in the late 1950s, but they were small scale (see www.aereoncorp.com). The company tested the "deltoid aerobody" shape, also called a deltoid pumpkinseed, with a 1,200-lb. manned demonstrator in 1970-71. That was followed by several studies funded by the military at less than $1 million. In the U.K., the now defunct Advanced Technologies Group, built a 40-ft.-long unmanned SkyKitten hybrid airship modeled after Lockheed Martins Aerocraft program and flew it in 2000 (AW&ST Sept. 23, 2002, p. 30). Nothing in the field has progressed to the size or apparent sophistication of the Skunk Works manned testbed.
The P-791 uses four air cushions as landing gear, located on the outer lobes. Taxiing the vehicle could be like flying a hovercraft, except one with greater exposure to winds. An advantage of the air cushions is they could be reversed to suck the aircraft onto the ground to resist winds for cargo operations.
GROUND HANDLING IS A MAJOR ISSUE facing hybrid airships but not hybrid aircraft. Conventional lighter-than-air airships require large ground crews and, because they are especially sensitive to winds on the ground, the airstrip is an area ripe for accidents.
The P-791 appears similar to the proposed full-scale version of the British SkyKitten, because it was a technology directly derived from Lockheed Martins earlier Aerocraft design. These low aspect shapes have a somewhat similar overall appearance--though the Skunk Works design is wider--and similar propulsion layouts, and both use air cushion landing gear. The vehicles look similar because the Lockheed Martin design was lifted, inappropriately from Lockheed efforts while the British company was employed as a consultant to Lockheed Martin. Legal infringement proceedings against the perpetrators has been anticipated and recently rumored by informed sources.
"Hybrid aircraft have been the subject of studies and questions for half a century," one expert says. "Now it stops being hype and they will meet reality."
The Hybrid Aircraft Corporation [[HAC Inc]http://www.hacinc.us] is the trademark owner of SkyCat(TM) which is a characterization of the technologies that comprise the hybrid aircraft vehicle as a unique class of aircraft.
